Various methods for using penetrating radiation such as x-rays in order to probe the contents of a sealed container such as airline baggage have been developed over the years. Some of these use the signature of x-rays scattered in various directions by material within the container. Since the cross-section for scattering is generally small, which is to say that only a small fraction of the incident beam is scattered into the acceptance solid angle of any particular detector, efficient strategies are necessary in order to discriminate threat items from the general background.
Furthermore, the detection of sheet explosive, concealed in the lining of suitcases or other containers, remains a particularly difficult challenge for traditional x-ray detection systems. This is because sheet explosives may be very thin, possibly no thicker than 2 mm. X-ray backscatter systems provide a means of detecting sheet explosives, but can generally only find a sheet when it is on the side of the container close to the backscatter detectors. Sheet explosives concealed in the lining on the other sides of the container will often not be found.
A further challenge to explosives detection is the concealment of explosives in electronic devices such as personal computers, radios, and cassette recorders, all of which make excellent hiding places for concealing explosives because an electronic device is not easily opened by security personnel for inspection and because electronic devices appear very cluttered in x-ray transmission images, making it easy to conceal an explosive with its associated detonator, trigger system, batteries, and wires within the electronic device.